refactor: use username variable consistently

Replaced hardcoded "h" username references with variables:
- hosts/vm: use username variable for secrets.username
- home configs: use username variable in bash initExtra paths
- keepassxc: update comment to use $HOME instead of /home/h

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

home/hosts/astyanax/default.nix

@@ -32,12 +32,12 @@ in
       enable = true;
       enableCompletion = true;
       initExtra = ''
-        for f in /home/h/.bashrc.d/*; do
+        for f in /home/${username}/.bashrc.d/*; do
           [ -f "$f" ] && source "$f"
         done
 
-        source /home/h/.bash_aliases/all
-        source /home/h/.bash_aliases/lang-js
+        source /home/${username}/.bash_aliases/all
+        source /home/${username}/.bash_aliases/lang-js
 
         # host-specific config goes here
         # ...